Within some therapy areas the tendency of a patient to adhere to the prescribed therapy is dependent on the simplicity of the specific treatment regimen. For example, many people with type 2 diabetes are diagnosed with the disease at a relatively high age where they are less prone to accept a treatment that intervenes too much with their normal way of living. Most of these people do not like constantly being reminded of their disease and, as a consequence, they do not want to be entangled in complex treatment patterns or waste time on learning to operate cumbersome delivery systems.
Basically, people with diabetes need to keep track of, and minimise, their glucose excursions. Insulin is a well-known glucose lowering agent which has to be administered parenterally to be effective in the body. The presently most common way of administering insulin is by subcutaneous injections. Such injections have previously been performed using a vial and a syringe, but in recent years so-called injection devices, or injection pens, have gained more and more attention in the marketplace. This is for one thing due to the fact that for many people these injection devices are easier to handle, particularly as they do not require the user to carry out a separate filling procedure before each injection.
In some prior art injection devices which are suitable for self-injection, the user has to set a desired dose using a dose setting mechanism of the injection device and subsequently inject the previously set dose using an injection mechanism of the injection device. In this case the dose is variable, i.e. the user must set a dose which is suitable in the specific situation each time a dose is to be injected.
Other prior art injection devices are adapted to inject a fixed dose each time it is operated. In this case the user has to prepare the injection device, thereby setting the fixed dose, using a dose setting or loading mechanism, and subsequently inject the dose using an injection mechanism.
U.S. Pat. No. 4,973,318 discloses a disposable syringe comprising a protective cap which is removably mounted over a first housing element of the syringe. The cap is configured to abut a second housing element while mounted in place on the first housing element. The protective cap is engaged with the first housing element such that rotation of the cap with respect to the second housing element causes rotation of the first housing element with respect to the second housing element. This relative rotation causes a variable dose to be set, i.e. the protective cap is used when setting a dose. However, it is necessary for the user to perform the step of setting a dose as well as the step of injecting the set dose.
U.S. Pat. No. 5,674,204 discloses a medication delivery pen having a medication cartridge, a pen body assembly and a cap. The pen body assembly includes a dose setting mechanism and a dose delivery mechanism that are selectively disconnected and connected by attaching and removing, respectively, the cap of the medication delivery pen. When the cap is attached to the medication delivery pen the user can easily dial in and correct the dialled in dosage and when the cap is removed the medication delivery pen is ready to dispense the dialled in dose. Thus, attaching/removing the cap to/from the medication delivery pen causes a clutch mechanism to be operated to switch the medication delivery pen between a dose setting mode and an injection mode. Also in this device it is necessary for the user to perform the step of setting a dose as well as the step of injecting the set dose.
An example from another medical device area, U.S. Pat. No. 7,302,948 discloses a nasal applicator in which a drug container is able to slide back and forth in response to the cocking and actuation of a spring. The drug container is slided backwards when a cap is attached to the nasal applicator. An abrupt stop during the forward movement of the drug container causes a piston to move and eject a dose of the drug through a dispensing nozzle.
U.S. Pat. No. 6,056,728 discloses an injection device which offers automatic needle insertion. It includes an intermediate chamber between the drug reservoir and the injection outlet for receiving a volume of the drug during preparation of the device for injection. The device has a rather bulky construction which makes it less attractive to carry around in e.g. a handbag.
It is desirable to provide an injection device which is simple to handle and which is intuitive and easy for the patient to learn how to use. In particular, it is desirable to provide an injection device which is capable of administering a number of doses of liquid drug, while at the same time requiring a minimum number of operations to be performed by the user. It is also desirable to provide an injection device which clearly indicates to the user when it is ready for injection and when the remaining volume of drug in the reservoir is insufficient to provide a full dose, and which then automatically renders further activation of the injection mechanism impossible. It is further desirable to provide an injection device which has a non-bulky design, so the user is not tempted to leave it at home instead of carrying it along during the day.
Some prior art injection devices offer so-called automatic delivery. These injection devices use energy from an internal energy source, typically a spring, to advance the piston in the reservoir. Automatic injection devices intend to reduce the force required by the user to eject the drug out of the reservoir. An example of such an injection device is found in U.S. Pat. No. 5,104,380.
In an automatic, spring-powered injection device where an engagement member is retracted axially along a toothed piston rod when the device is readied for injection, it must be ensured that the spring is cocked and secured against release at the same time as the engagement member moves into engagement with a dedicated tooth on the piston rod. If the engagement member has moved into engagement with a tooth on the piston rod but the spring has not been secured against release, the device will deliver an unintended dose. On the other hand, if the spring has been cocked and secured against release without the engagement member having moved into engagement with a tooth on the piston rod, no dose will be delivered when the injection mechanism is activated.
It is therefore desirable to provide an automatic injection device with which the user is ensured that a dose is either set correctly, and secured against delivery until the user activates the injection mechanism, or not set at all.
In U.S. Pat. No. 6,193,698 a spring is used to bias a dosing button and a drive arrangement towards a proximal position in an injection apparatus. During an injection the dosing button and the drive arrangement are pushed towards a distal position. To prevent uncontrolled injections, a locking member prevents return movement of the dosing button and the drive arrangement against the biasing force of the spring. In order to release the dosing button the user must manually press a trigger button which is accessible only after manual placement of two sleeves in a “zero” position relative to each other.
It is desirable to provide an injection device which locks the dosing button in a distal position following an injection, and which automatically releases the dosing button and moves it axially back into a proximal position when the injection device is readied for an injection so the user can see that the device is handled properly.
EP 1 304 129 discloses an injection device which includes a mechanism for automatically locking out the dose dial from an inadvertent injection after the dial has been retracted to set a dose. The lockout mechanism comprises an interference fit between flexible fingers formed in the dial and a groove in the device housing. These fingers must be able to withstand large compressive forces in order to prevent the dial from being depressed in case of misuse or accidental handling of the device.
US 2007/0135767 discloses another example of an injection device which includes a mechanism for preventing inadvertent depression of an injection button.
It is desirable to provide an injection device which the user does not risk inadvertently activating to eject a dose of drug while the protective cap is still on and which at the same time does not require a mechanical lock that is able to resist large forces.
It is further desirable to provide an injection device which is both safe and effective to use and safe to carry around.
Generally, when manufacturing injection devices which comprise a piston rod adapted to move a piston in a reservoir to thereby expel drug out of the reservoir it is essential that the piston rod is in engagement with the piston during the entire course of injection. If this is not the case the user may risk injecting a smaller amount of drug than intended. However, it is for several reasons preferred that the drug is not pressurised in the reservoir when the user takes the injection device into use for the first time. Injection devices are therefore often manufactured in such a way that a small clearance is deliberately provided between the piston rod and the piston to allow some play of the piston rod during transportation. In case of variable dose injection devices, when taking the device into use for the first time, the user sets a small dose and ejects it into the air. This action primes the injection device such that when a next dose is set the user is sure to inject the correct amount of drug, since the piston rod and the piston are now connected. In some fixed dose injection devices the piston rod travels a substantial distance each time a dose is injected. If the user in this case initially sets a dose and fires it into the air to prime the device, a substantial amount of drug may be wasted to the surroundings. This is particularly unwanted if the drug is expensive.
It is therefore desirable to provide a fixed dose injection device with which a user can perform an initial priming without wasting an approximately full dose of drug.